1. Field of the Invention
The present invention relates to methods for acquiring seismic energy signals and, more particularly, to such methods which are specialized to attenuate horizontally traveling seismic waves.
2. Setting of the Invention
In the acquisition of seismic energy signals that are used to locate subterranean oil and/or gas deposits, it is known to utilize linear arrays of seismic energy receivers, i.e., long in-line distributions of geophones. While these linear arrays of seismic energy receivers are especially good at attenuating, i.e., reducing or limiting, in-line noise (i.e., horizontally traveling seismic waves that travel parallel or nearly parallel to the linear array), these linear arrays are relatively ineffective at attenuating crossline noise (i.e., horizontally traveling seismic waves which travel nonparallel or oblique to the linear array) and where high velocity material is on the earth's surface or at the near surface. When using linear arrays, the crossline horizontally traveling waves sometimes cause the signal-to-noise ratio of the collected seismic traces to be so poor that the seismic traces are of little use.
Various processing methods and various seismic energy receiver array layouts have been tried to obtain better attenuation of these horizontally traveling seismic waves; however, these methods for practical purposes cannot greatly improve the quality of the resultant seismic energy traces which suffer from crossline noise. This is often the case where the backscattered coherent noise encompasses most of the recording spread, at the reflection times corresponding to the horizons of interest.
Some of the various array layouts tried in the past include a plurality of linear arrays of seismic energy receivers, i.e., a plurality of rows of seismic energy receivers. One such array layout is disclosed in U.S. Pat. No. 4,403,312 to Thomason. In Thomason, arrays of seismic energy sources and seismic energy receivers are used to increase the signal-to-noise ratio by increasing the total number of signals (traces) which are reflected from any one common depth point (CDP), which is well known in the art. Thomason accomplishes this by using a particular source-receiver geometry, where the receivers are on the perimeter of a box layout and the sources are in the interior of the box layout or vice versa. The recorded seismic signal traces are then summed together after assembling specified traces for a number of the specified and different source records. The configuration of the arrays of the seismic energy sources and seismic energy receivers is determined by trial and error to obtain the best signal-to-noise ratio.
Nowhere is it disclosed or suggested within Thomason to space the seismic energy receivers a predetermined distance apart in the crossline direction and to have a predetermined total crossline extent, this spacing specifically chosen to attenuate seismic waves of particular wavelengths. Further, it is not disclosed or suggested within Thomason to further attenuate horizontally traveling seismic waves by combining the seismic traces by sorting and summing (stacking) not during the acquisition of the seismic energy signals, but during the processing of the signals.
Further, high performance arrays have been tried, these arrays are defined as those arrays with substantially greater attenuation capabilities than unit weighted or merely linear arrays. The output of these arrays results in one recorded seismic trace which often fails to achieve the desired result of attenuating horizontally seismic waves because of three factors. First, it is highly unlikely that each element of the arrays, whether it be a single geophone or a single source point, will be identically coupled to the earth and this will result in earth coupling variations from geophone to geophone or from source point to source point, which will modify the effective weights applied to each element of the array and thus degrade the performance of the array. Secondly, environmental features (such as trees, boulders, streams, etc.) often cause the desired element spacing within the array to be physically unachievable. Again, this causes the performance of the array to be degraded. Thirdly, in many areas, the surface noise waves have long wave lengths that are within the desired seismic frequency band; thus the field arrays necessary to attenuate these long wave lengths can become so large that the desired reflection events can be compromised by intragroup statics or differential moveout within the group layout.
There is a need for a method for attenuating horizontally traveling noise waves in a manner that takes advantage of the particular spacing of groups of seismic receivers for the purpose of attenuating seismic waves with a certain wavelength. Further, there is a need for a method which during the process attenuates horizontally traveling seismic waves to a greater extent capable than in prior art methods.